1. Technical Field
Embodiments of the present disclosure are related generally to hybrid vehicles, and in particular to those that employ a drivetrain that includes a multi-gear transmission.
2. Description of the Related Art
Motor vehicles are commonly powered by an internal combustion engine (ICE) that is mechanically connected to the drive wheels through a multi-gear transmission. A multi-gear transmission is characterized by the availability of multiple distinct gear ratios, or “gears”, at which it may transmit mechanical power from the engine to the wheels.
Gear ratio is commonly defined as the ratio of input speed to output speed. In a conventional motor vehicle, input speed is simply the engine speed, while output speed relates to the wheel speed or vehicle speed (possibly as modified by an additional gear ratio of a differential or final reduction gear residing downstream). To determine output speed, input speed is divided by the gear ratio. To determine output torque, input torque is multiplied by the gear ratio. Therefore the speed of the engine and the torque output to the wheels may be controlled by selecting among the various gear ratios offered by the transmission.
The gear ratios offered by a multi-gear transmission are commonly numbered sequentially, from a lowest-numbered (or “low”) gear at which the gear ratio is the largest of all the gears, to a highest-numbered (or “high”) gear at which the gear ratio is smallest. The present application follows this convention, thus reference to a “lowest-numbered” gear means the gear at which the gear ratio is the largest of all the gears, etc. Shifting the transmission from a given gear to a higher-numbered gear is referred to as an upshift, and shifting from a given gear to a lower-numbered gear is referred to as a downshift.
It follows that at a given vehicle speed, a low-numbered gear is associated with higher engine input speeds and higher torque outputs, and a high-numbered gear is associated with lower engine input speeds and lower torque outputs. The scheduling of upshifts and downshifts therefore affects the performance and efficiency of the vehicle. The practice of delaying an upshift until relatively higher engine speeds are reached tends to promote torque availability at the wheels, but causes the engine to run at relatively higher and less efficient speeds. Similarly, advancing an upshift to occur at lower engine speeds tends to cause the engine to run at relatively lower and more efficient speeds, but reduces torque availability at the wheels.
Automatic and automated manual transmissions are automatically shifted to a selected gear number by a shift controller. The gear number is typically selected by reference to a shift schedule. A shift schedule is a rule that relates a set of operating regions to a set of target gear numbers. Operating regions are typically defined in terms of vehicle speed and throttle position. The assignment of a specific gear number to a specific operating region determines the scheduling of upshifts and downshifts and therefore determines vehicle performance and fuel efficiency. A shift schedule may be designed to balance these conflicting objectives as well as possible, or to favor one objective over the other. Devising a single shift schedule that properly balances these conflicting concerns can be a challenge.
When designing a shift schedule, it should be taken into account that every gear shift is likely to be accompanied by a brief torque interruption to the wheels, a noticeable change in acceleration, and a change in speed of the engine, all of which may be noticed by the driver and detract from overall feel and drivability. For these reasons shifting should occur only where the performance or efficiency benefits justify the effect on drivability.
A shift schedule should also account for the fact that in some circumstances, such as under high load, shifting to a higher gear number may prevent the vehicle from further accelerating or even maintaining its speed. This is because the smaller gear ratio associated with the higher gear number reduces torque output at the wheels. The resultant increase in necessary input torque from the engine may approach or exceed its maximum torque output at the indicated engine speed.
Similarly, shifting to a lower-numbered gear also presents a possibility of overspeeding the engine. This latter concern is lessened to some degree by the inherent inertia and friction of an internal combustion engine, which may result in an engine braking effect, preventing runaway. However, this concern remains important for small engines of low inertia, or drive means such as electric or hydraulic motors which have much lower inertia than a typical ICE.
Beyond these concerns, the primary goal of a shift schedule is to provide desired performance at an acceptable efficiency. Because the efficiency and maximum torque of an ICE can vary significantly with operating speed, the more gears that are available the more effective an operating point can be found. Also, the relatively large inertia of an ICE imposes practical limits on the allowable speed change between gears, and the allowable speed range within a given gear, therefore favoring gear changes of a relatively small magnitude. These factors have tended to encourage the use of a transmission with four to six distinct gear ratios, and a shift schedule that utilizes each available ratio in sequence.
Best practices for developing a shift schedule for a transmission connected to an ICE on a conventional vehicle are well understood, and most shift schedules so developed are well adapted to the specific behavior and limitations of an ICE. The design of shift schedules for hybrid vehicles is not as well established. In particular, series hybrid vehicles that include a multi-gear transmission driving the wheels are likely to pose a different set of issues and limitations to the design of a shift schedule.
For example, with regard to shifting, the primary difference between conventional and hybrid vehicles is related to the way in which the internal combustion engine (ICE) is utilized. A series hybrid vehicle utilizes a secondary drive means to drive the wheels, such as a hydraulic or electric motor. The engine is freed from driving the wheels, allowing it to follow a more optimal duty cycle that is independent of the power demanded at the wheels. Having lower inertia than an ICE, a hydraulic or electric motor may take on a greater change in speed before impacting the noise or vibration of the vehicle, meaning that a larger range of operating speeds can be served by a single gear ratio. Similarly, the lower inertia may support a larger allowable speed change to result from a shift, meaning that a larger difference between each sequential gear ratio may be accommodated. Finally, the efficiency of an electric or hydraulic motor tends to be much less sensitive to speed and load than an ICE, which means that an equivalent degree of speed and torque optimization may be achieved with a smaller number of gear ratios. These factors mean that the drive means may in many cases operate without a multi-gear transmission, and may instead have a single-gear transmission, or a dual-gear transmission operating on very simple shift schedule.
On the other hand, there remain opportunities to optimize the efficiency and cost of a hybrid vehicle, while maintaining acceptable torque loads and response, by including a multi-gear transmission with an appropriate shift schedule. For example, an electric motor driving through a single-gear transmission will encounter significant portions of its duty cycle at efficiencies far below the peak efficiency, particularly at low speeds and loads. Sensitivity to speed and load, although diminished, is still a significant factor, and may benefit from retaining a choice of multiple gear ratios at which to operate the motor.
It is known to define a plurality of predetermined shift schedules each favoring a different balance of performance and efficiency, or each being optimized for different loading conditions, for automatic selection by a controller in response to sensed loading or other conditions for conventional vehicles. For example, the load or mass of the vehicle, or the road grade, may be sensed by a sensing means and fed to a controller that chooses a schedule designed to maintain performance when the load is particularly large or the grade is steep. Or, when the vehicle is lightly loaded, a shift schedule optimized for economy might be selected by the controller. However, applicant is unaware of any prior attempt to optimize the use of multiple shift schedules with a series hybrid vehicle.